The future is underground - from open-pit to underground mining

Materials World magazine
,
1 Aug 2011
Mine at Chuquicamata, Chile

Keith Marshall, Global Practice Leader of Underground Mining
for Rio Tinto, discusses the opportunities but also the risks
associated with the inevitable transition from open-pit to
underground mining.

Underground mining is a much more
complicated process and has a significantly
higher risk profile than open pit mining, but
the production from open pits is declining and the
answer is to move underground. And these new
underground mines will be much more productive
than existing operations, awareness of the increased
risks associated with what I see as an inevitable
transition to underground mining is very important.

Typically, underground mines have been much
smaller than open pits, with much lower output and
higher unit costs, but usually with a significant grade
premium. All that is about to change as we look
to develop large block cave mines that can compete
directly with open pits in terms of output and unit
cost, while still operating with some degree of grade
premium. Scale is crucial as the demand for
metals, particularly copper, has been on an upward
trajectory for decades. Copper consumption has
risen in line with global population over the past
50 years, reaching 16.2Mt in 2010. Today the USA
copper consumption is around 10kg per capita per
annum, while in China and India it is less than 2kg. I fully expect this to change as the populations of
China and India strive to establish their own urban
populations and significantly increase their per capita
copper use.

One third of this copper comes from Chile, hosted
in large-scale low-grade copper porphyry deposits,
with Peru (1.28Mt) and the USA (1.12Mt) being the
second and third largest producers, and also with
similar porphyry deposits. Some of these mines have
been in open pit production for more than 100 years,
including the largest open pit (by total production) in
the world, Chuquicamata in Chile, and they are now
looking to follow the mineralisation underground.

Increasing output in an open pit mine is
predictable. Ramping up production, increasing
schedules, recovering more metal and lowering
operating costs can be achieved with more
equipment and more people. However, a number of
open pits are reaching depths of 1,000m and are
considered mature. There is a limit to the depth that
pits can go due to geotechnical and operating
constraints, and especially the risks concomitant with
excessively high pit walls. Furthermore, the number
of new ore discoveries at depth has increased
compared to those at or near the surface, where
there have been few, and for the great majority of
those new discoveries the grade of the ore was lower.

Increasing production in an underground mine is
not as easy as in an open pit because there is
often limited access and working areas. The
consequence is that project and production slippage
is usually unrecoverable, and therefore the
underground projects appear to perform badly
against expectations. Almost without fail, the
expectations that are set at the project phase
are unrealistic.
Transitional changes
The transition to underground mining will bring a
number of challenges, the largest of which is to
replicate the production capacity achieved in open pit
mines, where production constraints are less of a
challenge or non-existent. Over the last 100 years, production from underground mines has slowly
increased. Early methods, such as shrink stoping,
had a daily output of tens of tonnes per day. Greater
production, measuring hundreds of tonnes per day,
was achieved with longhole stoping, a degree of
automation, drilling and blasting holes up to 30m and
using load-haul-dump (LHD) machines and trucks.
These methods were surpassed by block caving,
which can deliver thousands of tonnes of ore per day.
Historically, caving was used at operations where the
orebody was low strength, caving readily and having
fine fragmentation. Over the years, advances have
been made in our understanding of the caving
process, together with the development of fit-forpurpose
equipment and the availability of reliable
cost models. These advances have allowed more
competent orebodies to be caved.

The precursor to modern block caving developed
during the late 19th Century in the iron ore mines in
northern Michigan in the USA. By the early 20th
Century, the method was being used in copper mines
in the western states and by the 1920s had
been introduced into Canada and Chile. The
southern African diamond mines and then asbestos
mines adopted the method in the late 1950s. A
new era of caving began in the late 1960s and
early 1970s with the introduction of LHD vehicles
and the beginning of modern, highly productive
mechanised caving.

Block caving is conceptually simple. Instead of
breaking rock using drill and blast, gravity is
employed. This lowers operating costs compared
with other methods. High production rates can be
achieved through economies of scale and a high
degree of mechanisation. As personnel are removed
from the rock breaking, it is a safer method. It relies
on undercutting the ore body at the extraction level
and the construction of inverted cones or drawbells
using drill and blast. Often strengthened with
reinforced concrete, these points will remain in
operation while ore is removed by LHD vehicles,
and in a large ore body this can be as long as
20-25 years. Once all is in place, the fracturing of the
overlying orebody is initiated by blasting.

The ability to create a cave in an orebody depends
on the strength of the rock and the size of the
undercut. A cave is initiated when an undercut
reaches a critical dimension. At this point, the
stresses induced by removing support for the
overlying rock mass exceed the inherent strength of
the rock mass. Following initiation, the cave can
be propagated in two planes – horizontal and vertical.
In the horizontal plane, it is controlled by the rate at
which undercutting takes place. Where it is not
necessary to undercut the entire orebody to achieve
the desired mining rate, a section or panel is caved.
Behind the active panel, reserves are exhausted,
while in front are virgin reserves not yet developed. In
the vertical plane, propagation is governed by the rate at which ore is drawn. If undercutting or draw
proceed too slowly, propagation can cease and the
cave may compact. On the other hand, inducing a
cave to propagate at too rapid a rate can lead to
stress damage to the permanent mine infrastructure.

Possible danger

Block caving is often selected as the underground
mining method of choice due to its low operating
cost. Technically, the major risk is the inability to stop
or restart the process once initiated. The mine rate
controls the speed but not always the result, as overlarge
fragmentation can be expensive in local rock
breaking, while the propagation of too many fines can
result in an outpouring at the drawbell. Another
danger is cavitation, leaving behind a void that later
collapses, forcing air, mud, water or a mixture
of these through the drawbell, often engulfing
machinery. Because these mines are designed as
large long-term operations, funding based on net
present value is challenging as the calculation period
can extend for decades, beyond any sensible
discount of process or costs. This requires education
and risk assessment for elongated time frames
by investors.

The move to block caving is underway. With
relatively low operating costs and daily mining rates
of 10-100,000t/day, it compares financially very
favourably with other mining methods, so long as the
method is applicable to the orebody in question.

Looking to the future

I believe that block cave mines are the future for our
industry. But we must not forget that these mines are
technically and financially challenging. Investment
decisions in block caving normally have to be taken
on limited orebody knowledge, and hence with an
increased level of risk. And block caving is often
selected as the underground mining method of
choice due to its low operating cost, but everyone
should be aware that the cost advantage comes with
a high risk premium.

Further information

www.riotinto.com